1. Figure S1: Observed May SST difference (ºC) between 9 coldest years and the 9 warmest years based on SST Anomalies. The dotted areas denote the statistical significance at the α=0.01 level of t-test values.

2. CAMS Surface Temperature WRF-NMM surface Temperature
Figure S2. Mean surface temperature (ºC) and precipitation (mm day-1) for June 2011 from CAMs and GTS observations and WRF-NMM simulation.
CAMS Surface Temperature
WRF-NMM surface Temperature
GTS Precipitation
WRF-NMM Precipitation

3. Figure S3. Observed/WRF simulated anomaly/difference of precipitation (mm/day) for July. (a) Observed; (b) SUBT effect; (c) SST Effect; (d) SST effect plus SUBT effect. The dotted areas denote statistical significance at the α=0.01 level of t-test values. a. b. c. d.

4. Figure S4. Simulated May geopotential height (GPM) difference at 500 hPa between Case 2011 and Case SUBT.

5. Figure S5. Perturbation pathway: temporal-zonal cross section of vorticity anomaly (106 s-1) at 500 hPa averaged over 35N and 50N.

6. Figure S6. Observed/WRF simulated anomaly/difference of surface temperature (ºC) for May. (a) Observed; (b) SUBT effect; (c) SST Effect. The dotted areas denote statistical significance at the α=0.01 level of t-test values. a. b. c.

7. Table S1. Observed differences between year 2011 and the benchmarks, and GCM-simulated differences for different scenarios